Sacrificial anode system for protecting metals in sea water



Oct. 9,k 1951 H. A. ROBINSON ET AL SACRIFICIAL. ANODE SYSTEM F'OR PROTECTING METALS IN SEA WATER Filed June 15, 1949 .figg .5

IN V EN TORS Haro/d Ro/'nson H//ary A. #Umb/6 A TTOR/VEYS Patented Oct. 9, 1951 SACRIFICIALANOE SYSTEM FOR PRO- TEC'IING METALS INSEA WATER Harold A. Robinson and Hilary A.Humble, vMidland, Mich., assignors to TheDow Chemical Company, Midland,.-Mich., a corporation of Delaware Application Ilune 15, 1949,-SerialN0.--99,156

'IY-his'invention relates to a sacrificial anode system forprotecting corrodible metal structures immersed in seawater.

The tendency for structures of steel and .similar metals, when immersed in seawater, to :j

undergo serious corrosion can be offset by oathodic protection. In this process the struc- 4ture -iszmade the cathode in an electric circuit vusing ,the seawater as electrolyte. If suflicient '-.current is supplied, the structure can be kepl- `from corroding.

Such-cathodic protection in seawater, while Ieiiective, poses -a real practical problem in that the current requirements change drastically Vwith time, making it diicult to design an ecoy .nomical current supply system. If the initial current is su'icent to protect the structure com- Y pletely from the outset, in a short time that current far exceeds the l.actual requirements, due to polarizationand the deposition of desirable. calcareous coatings of the type described in U. S.

Y2,200,469. On the other hand, if the initial current is of the order of that required when equi- .librium is ultimately reached, months and even vyears may elapse before complete control of corl rosion is obtained. Consequently, the designer must choose between installing a large electrical supply system, most of the capacity of which soon becomes unnecessary and represents an economicwaste, and installing a minimum sys.

tem, .thus risking a long period of inadequate protection. The intermediate choice, that of Aproviding a temporary source of high current in the initial stages, Ahas not been a possible one because of the engineering diculties and huge capital outlayv involved in making portable generati-ng equipment of the low-voltage high-cur- -rent type available -and transporting it from site .fhavio'r of magnesium metal ywhenoperating. as

an anode in seawater. In this medium, mag- 4 Claims. (Cl. 204-197) nesium metal yfunctioning anodicallycorrodes .uniformly at good electrochemical .efciencyand undergoesvlittle. if any polarization or coating. Extremely largecurrents can .belgenerated by..dis

posing vthe magnesium -metal insuch a. shapelas` to have a high surface area per unit weight, ,and those .currents can be withdrawn continuously, without signicant .decrease in magnitude vor voltage, .until the magnesium metal is almost completely exhausted. These qualitiesrender .magnesium metal uniquely suited to use` in the .galvanic protectionof corrodible metalswin, seawatenand permtthe design ofaninexpensive. easily. installed protectivesystem.

A further. discussion, by one of. the present applicar1ts,.of thev phenomena involved inY protecting steel .cathodically in seawater-,and of the behavior VVof magnesium as an .anode in .that medium, has recently appeared in Corrosion, vol. 4, pages 358-370 (July 1948).

.TheprOteCtiVe System of the present invention essentially .comprises two distinct current sourcesfboth connected to .the structure to be protected. so Vas to render it .cathodic. Theirst or Yprnciary source.. comprising high-output maenesium anodes is a temporary .one designed to provide an initial current suicient toprote'ct the. structure at onceand to polalze .ill C0111- -pletely within a fewdays, and then to become inoperative through exhaustion when this initial stage has been completed. The other or secondary source is a long-time one for providing continuously the smaller equilibrium current necessary to maintain the structure protected overaperiod of months or years. This second source also l, preferably comprises magnesium vanodes but of relatively much lower rate of output .than those of the first source.

As stated, the first current source of the system o f 4the invention consists essentially of vrapidly-expandable magnesium metal anodes submerged in the seawater and connectedfelectrically to the structure to bevprotected. The .shape `of these anodes must be such that'they provide throughout their short life an extremely highcurrent per unit weight of magnesium. `To

tthis end, a surface area of at least 0.5 square foot per pound .of magnesium metal is desirable, ,with .areas of 0.5 to V1.3 square feet per pound being preferred. The most'convenient shape conv4sistentwith this area limitation is that of a thin vrilexilole ribbon o r tape.

Inv order to insure electrcalunity during usegthe ribbon should have a continuous core of metalcathodicto the'magnesium metal, e. g. steel wire. In practiceya magnesium metal ribbon having a rectangular cross-section of inch by 3A inch and a core of No. 13 gauge iron wire is highly satisfactory. Such ribbon weighs 0.22 pound per foot and has a. surface area of 0.85 square foot per pound. In service at the electrochemical eiciency observed in seawater, it furnishes before exhaustion slightly more than 100 ampere-hours per lineal foot.

Since the function of the primary high-surface-area anodes is to provide for the initial protection and polarization of the structure to be protected, it is essential that the mass of expendable metal be sufiicient to develop the necessary quantity of electricity. To this end, the weight of ribbon or other high-area metal should correspond to at least 0.012 pound of magnesium metal per square foot of structure to be protected. However, weights much in excess of this value are unnecessary, since it is desirable for maximum ,economy that the high area metal be completely consumed at about the time the structure becomes fully polarized. As a practical matter, Weights of 0.012 to 0.036 pound of magnesium metal per souare foot of surface to be protected are ordinarily used.

With primary magnesium metal anodes of the surface area and weights just set forth, the structure to be protected receives an initial current of from 30 to 60 milliamneres per square foot for a period of about 5 to 15 days. At the end of this time, the structure will be well polarized and will have also developed a calcareous cathodic coating. The primarv anodes will be almost completely expended and will cease to develop appreciable current. However, with the structure thus in equilibrium condition, it can be maintained protected by the very much smaller current provided by the secondary current source of the in- Aa. capacity suiiicient to supply this equilibrium lcurrent, with some reserve being desirable. Further, the source should be capable of delivering this current continuously for a period 'which is at least a substantial fraction of the useful life of the structure, certainly for a year or two and preferably much longer.

'I'he secondary source may, if desired, be of the conventional generator or rectifier type. However, magnesium metal anodes are effective for this purpose also. provided they are suiiiciently massive to have the necessary life and are of low surface area so that excess current is not developed. In general, slowly-expendable magnesium metal anodes should have such a shape as to have a surface area less than 0.1 square foot per pound and of such total weight as to correspond to at least 0.035 pound of magnesium metal per square foot of structure to be protected. For electrical integrity, each anode is desirably constructed with a continuous core of a metal cathodic to the magnesium metal, e. g. a steel rod or pipe. A highly satisfactory form is that of a cast cylindrical block having a diameter of 8 inches and a length of 16 inches, with a central inch I. P. S. iron pipe core. Such an anode Weighs 51 pounds, has a surface area of 0.07

square foot per pound, and, in operation in sea- Water, develops slightly over 3 ampere-years.

For a further explanation of the present invention, reference will be made to the accompanying drawing showing the details of a preferred cathodic protection system embodying the principles already discussed. In the drawing,

Fig. 1 is an elevation, partly in section along the line I-I of Fig. 2, illustrating a steel sheet pile being protected by magnesium metal ribbon as primary current source and massive magnesium metal anodes as secondary source;

Fig. 2 is a reduced plan view of the installation of Fig. 1;

Fig. 3 is an enlarged cross-section through the magnesium metal ribbon;

Fig. 4 is an enlarged detailed view, in perspective, of the manner of attaching the magnesium metal ribbon to the piling; and

Fig. 5 is an enlarged vertical section through a. massive magnesium anode, illustrating the electrical connection.

In the drawing, the system of the invention is being used to protect a vertical steel sheet pile bulkhead 6 immersed in seawater 1. Since the bulkhead may be of great length, only a fragment is shown. The primary current source for initially polarizing the structure is a magnesium metal ribbon 8 having a steel wire core 9. Lengths of the ribbon 8, bent into serpentine form, are attached near the top of the pile 6 above water level at spaced intervals along the bulkhead and hang vertically downward into the seawater. As shown in Fig. 4, each ribbon 8 is connected firmly to the pile 6 by means of a screw-clamp IIJ. To insure low electrical resistance, the ribbon, clamp, and piling should all be clean and bright at the point where the connection is made. The dimensions of the individual ribbons 8, and their number and spacing along the bulkhead, are chosen to provide a surface area and weight of magnesium metal within the limiting values for the rapidlyexpendable primary current source already explained.

The secondary or long-term maintenance protective current is provided by massive cast magnesium metal anodes II, each provided with a steel pipe core I2. As shown in Figs. 1 and 5, a group of these anodes is strung on an electrically conducting cable I3, the core of each anode being fixed mechanically and electrically to the cable by a weld I4. The cables I3 and the attached anodes are swung from eyebolts I5 mounted on steel brackets I6 welded to the pile 6 at intervals along its length. The cables are connected electrically to the brackets I6 by waterproofed low-resistance joints I'I. The dimensions of the cast anodes and their number and the number and spacing of the brackets are likewise chosen to provide the surface area and weight of magnesium metal within the values for the secondary current source previously set forth.

While the system of the invention has been described primarily as applicable to the protection of steel in seawater, it is equally effective in protecting immersed structures of other corrodible metals no more anodic than zinc, e. g. galvanized ironand copper. The system is useful in protecting structures not only in seawater of normal salinity, but also in the diluted water found in many Seaports and in the concentrated brines produced by evaporation of seawater. The term magnesium metal as used herein refers to magnesium and to magnesium-base alloys containing at least per cent by weight Of. magnesium.

What is claimed is:

1. In combination with a structure immersed in seawater and formed of a corrodible metal no more anodic than zinc, a protective system of sacriicial metal submerged in the seawater near the structure and comprising a rapidlyexpandable quantity of magnesium metal of such shape as to have a surface area of at least 0.5 square foot per pound and such mass as to correspond to at least 0.012 pound per square foot of structure to be protected and a slowly-expendable quantity of magnesium metal of such shape as to have a surface area less than 0.1 square foot per pound and such mass as to correspond to at least 0.035 pound per square foot of structure, all such magnesium metal being connected electrically to the structure.

2. In combination with a steel structure immersed in seawater, a protective system of sacricial metal submerged in the seawater near the structure and comprising a rapidly-expendable quantity of magnesium metal ribbon having a surface area of at least 0.5 square foot per pound and such mass as to correspond to from 0.012 to 0.036 pound per square foot of immersed steel structure disposed close to the structure, and a slowly-expendable quantity of massive magnesium metal having a surface area less than 0.1 square foot per pound and such -mass as to correspond to at least 0.035 pound per square foot of structure disposed more remotely from the structure than the ribbon, al1 such magnesium metal being connected electrically to the structure.

3. A system according to claim 2 wherein the magnesium metal ribbon and massive magnesium metal are each provided with a continuous core of metal cathodic to the magnesium metal. 4. In combination with a sheet steel structure immersed in seawater, a protective system of sacricial metal submerged in the seawater near the structure and comprising a plurality of lengths of steel-cored magnesium metal ribbon having a surface area of at least 0.5 square foot per pound paralleling the structure, the total mass of such ribbon corresponding to from 0.012 to 0.036 pound of magnesium metal per square foot of immersed steel structure, and a plurality of massive steel-cored magnesium metal anodes each having a surface area less than 0.1. square foot per pound, the total mass of such anodes corresponding to at least 0.035 pound per square foot of structure, the said ribbons and anodes being connected electrically with the steel structure, the ribbon being so placed relative to the structure as to develop a current density at the latter of from 30 to 60 mlliamperes per square foot and the massive metal being so placed as to develop at the structure a current density of from 2 to 5 milliamperes per square foot.

HAROLD A. ROBINSON. HILARY A. HUMBLE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,200,469 Cox May 14, 1940 2,444,174 Tarr et al. June 29, 1948 2,478,478 Grebe Aug. 9, 1949 

1. IN COMBINATION WITH A STRUCTURE IMMERSED IN SEAWATER AND FORMED OF A CORRODIBLE METAL NO MORE ANODIC THAN ZINC, A PROTECTIVE SYSTEM OF SACRIFICIAL METAL SUBMERGED IN THE SEAWATER NEAR THE STRUCTURE AND COMPRISING A RAPIDLYEXPANDABLE QUANTITY OF MAGNESIUM METAL OF SUCH SHAPE AS TO HAVE A SURFACE AREA OF AT LEAST 0.5 SQUARE FOOT PER POUND AND SUCH MASS AS TO CORREPOND TO AT LEAST 0.012 POUND PER SQUARE FOOT OF STRUCTURE TO BE PROTECTED AND A SLOWLY-EXPENDABLE QUANTITY OF MAGNESIUM METAL OF SUCH SHAPE AS TO HAVE A SURFACE AREA LESS THAN 0.1 SQUARE FOOT PER POUND AND SUCH MASS AS TO CORRESPOND TO AT LEAST 0.035 POUND PER SQUARE FOOT 